You may have to investigate how to install and run applications from faster external storage.
Should that be among your options; I'd try to upgrade internal, that'd work out better.
Extra flash-based storage or an SSD as external, may be useful. ~ There would be rules and
a learning curve; for example you'd not be able to close the apps or disconnect external HD
without un-mounting first.
Depending on the build year model, and internal configuration, there may (likely) be upgrade
for the internal storage. An older unit may not have options via Apple service; could have one
through third party. Check with 'macsales' for internal options; they have also externals.
Look to their site and see if there's a hardware upgrade match for your MB/Air's internal storage.
Battery life may suffer, with external SSD or flash based drive. Study an 'internal upgrade' solution.
Good luck & happy trails!🌞